1. Field of the Invention
This invention relates to ion implantation of semiconductor devices. More particularly, this invention relates to ion implantation techniques that are suitable for ion implanting vertical cavity surface emitting lasers and that can result in novel implantation structures.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of conductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to emit different wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 (another confinement layer) is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26. Alternately, the top mirror and graded-index region can consist of a tunnel junction structure. This comprises a short p-doped region nearest the active region junction. Beyond the p-doped region is a tunnel junction followed by an n-type DBR.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonate at specific wavelengths, the mirror separation is controlled so as to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 formed by implanting ions (protons or certain other elements such as deuterium, helium, iron, etc.) that provides current confinement. Protons can be implanted, for example, in accordance with the teachings of U.S. Pat. No. 5,115,442, which is incorporated by reference. Alternatively, the insulating region 40 can be formed using an oxide layer, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. However, the principles of the present invention relate to insulating via an ion implanting process. In either event, the insulating region 40 defines a conductive circular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type (or n-type in the case of a tunnel junction device) electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. The lower mirror stack 16 and the top mirror stack 24 are very high reflectivity mirrors, with the lower mirror being at least slightly higher reflectivity than the top mirror. Due to this difference, some photons emerge as coherent light 23, i.e., laser. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10. In the tunnel junction device format, the light follows the same path but through the tunnel junction layer and the n-type top mirror.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions (briefly described above), can be added.
In addition, it should be noted that multiple VCSELs can be formed on the same substrate, thus producing a VCSEL array. Ion implanting provides a method of electrically isolating individual VCSEL elements. To do so, ions with certain insulating interaction characteristics such as protons are implanted between the individual VCSEL elements to produce high-resistance zones that electrically isolate the VCSEL elements. Thus, by controlling ion implantation locations and energies, electrical confinement within a VCSEL and electrical isolation between adjacent VCSELs on the same substrate can be implemented.
Prior art ion implantation techniques usually direct ions perpendicularly or nearly perpendicular onto the surface of a wafer being implanted. While generally successful, such prior art implantation techniques are less than optimal in some applications. For example, perpendicular implantation is not suitable for laterally implanting large vertical aspect ratio mesa structures. Another limitation is the difficulty of implementing desired gain guide isolation steps. Finally, prior art ion implantation techniques induce substantial lattice damage in a device's aperture or electrical contact region. Such lattice damage can be highly detrimental to electrical or optical performance.
Because of the foregoing problems, a new ion implantation technique would be beneficial. Particularly beneficial would be an ion implantation technique that is suitable for use with devices having large vertical aspect ratio mesa structures. Also beneficial would be an ion implantation technique that avoids or reduces the problems related to obtaining required gain guide isolation steps. Also beneficial would be an ion implantation technique that reduces lattice damage in the aperture or electrical contact region near the aperture.